Autofocusing of related art used in an optical microscope is broadly classified into two methods: a passive method and an active method. In the passive method, a specimen is kept in focus, for example, by detecting a blur based on a change in the position where the contrast of an observed image of the specimen is maximized and performing feedback control on a z-axis stage or an objective lens drive unit to change the positional relationship between an objective lens and the specimen in the optical axis direction (z direction). When the specimen has low luminance or contrast, however, it takes time to perform the image analysis, resulting in a delay in focus adjustment or a focus error. On the other hand, the active method, a representative example of which is a slit projection method, is capable of quick focus adjustment because the z position of a specimen is detected based on reflection of infrared light or any other light with which the specimen (such as a cover slip) is irradiated. The two methods, however, which are capable of correcting a shift in the z direction, are not capable of correcting a shift in a direction perpendicular to the z direction (x and y directions).
Advances in super-resolution technologies and other similar technologies in recent years are drawing attention to microscopic observation in a nanometer scale, which is smaller than the resolution of an optical microscope. In observation in this resolution scale, a change in the position of a specimen being observed due, for example, to a mechanical drift of an apparatus and thermal expansion of the cover slip, which has not been a problem, can be an unignorable problem. It is therefore desired to develop a technology that allows the positional relationship between an objective lens and a specimen to be maintained three-dimensionally with precision. For example, in an apparatus described in Non-patent Literature 1, an active method is combined with image analysis for three-dimensional drift correction. In the apparatus, a z drift is corrected based on the active method, and x and y drifts and a residual z drift are corrected by analyzing time-lapse images.